The present invention generally relates to display devices, and more particularly, to a method and apparatus for controlling luminance of light emitting display devices.
The dashboard displays of modem transportation equipment: automobiles, aircraft, ships, trains, even the space shuttle, etc., require efficient, uniform illumination in order to accommodate operation under low ambient lighting conditions, e.g., evening or nighttime hours. The illumination should be made variable and controlled manually or automatically via a closed-loop sensor controlled feedback system, to accommodate changes in available ambient light.
Early attempts to satisfy these requirements employed rheostatically controlled incandescent lamps and even light emitting diodes, for instance. In other words, the lamps were dimmed by dropping power across an adjustable resistive element in series with the lamps. This technique is both inefficient and unreliable as a significant amount of power is dissipated in the rheostat which, in turn , leads to a very limited service life. Also, most mechanical devices of this type have numerous other xe2x80x9cnuisance failurexe2x80x9d mechanisms, e.g. wear, dirt contamination, open resistive element, to name just a few.
Controls of the type aforementioned typically have an exponentially decaying power curve associated therewith. This non-linear effect causes the lamp to dim quickly at first, then at an ever-decreasing rate, as the rheostat is varied. Conversely, the lamp brightens very slowly at first and then suddenly becomes very bright. Neither of the foregoing situations is desirable, especially to an aircraft pilot. In an attempt to eliminate or dampen the aforedescribed situation, one solution may be to use an xe2x80x9caudio taperxe2x80x9d rheostat (exponentially varying resistive element) in conjunction with the lamps. However, these devices are expensive, difficult to obtain, and have very limited resistance ranges.
Another technique for controlling the luminance of lamps employs the use of pulse width modulation (PWM). Pulse width modulation is well known in the art and allows the lamps to be turned on and off at a set frequency utilizing a variable duty cycle: the duty cycle being defined as the ratio of the xe2x80x9conxe2x80x9d time to the xe2x80x9ctotalxe2x80x9d time. Using PWM techniques, the duty cycle can easily be varied from zero (fully off) to 100 percent (fully on).
The PWM technique provides a significant improvement over the variable series resistive element described above. It is very efficient, and virtually no power is dissipated in the control potentiometer. Pulse width modulation techniques are well known to those skilled in the art and will not be discussed here. The problem with the PWM technique, although not as pronounced as with the series rheostat approach, is the non-linear output produced. The lamp power, or luminance, is actually the root-mean-square (RMS) of the pulse train presented to the lamps. Again, the effective, or RMS, values of periodic waveforms are well known and will not be discussed herein. Suffice it to say that the lamp illuminates much faster than desired, and conversely, extinguishes much slower due to the non-linearity of the lamp power associated with the RMS values of the pulse train presented to the lamps.
In view of the foregoing, it should be appreciated that it would be desirable to provide methods and apparatus for delivering linear power drive to a light source. Furthermore, additional desirable features will become apparent to one skilled in the art from the drawings, foregoing background of the invention, following detailed description of the invention.
An apparatus is provided for delivering linear power drive to a light source. The apparatus comprises a pulse width modulator responsive to a command voltage drive signal for providing a pulse width modulated signal at an output. The widths of the pulses and the frequency varying simultaneously as the command voltage drive signal is varied, and the output of the pulse width modulator being coupled to the light source. The apparatus also comprises a frequency control circuit responsive to the command voltage drive signal for supplying a control current to the pulse width modulator such that the frequency of the pulse width modulated output signal is varied.
In addition to the apparatus for delivering linear power drive to a light source, a method is provided for delivering linear power drive to a light source. The method comprises the step of providing a pulse modulation signal with the widths of the pulses thereof being varied in accordance with a command signal. In addition, the method comprises the step of varying the frequency of the pulse modulation signal simultaneously with the widths of the pulses being varied.